Replace use of SETTINGS to check compute resource availability
config/application.rb uses the global SETTINGS variable to store a list of compute resource bundler groups that are loaded, which is used later in ComputeResource to list available providers. The initialiser tries to load Bundler groups, then stores a per-provider "setting" based on whether the Fog provider is registered.
The listing of available providers in ComputeResource can be refactored to check the Fog::Compute.providers registration and remove this data from the settings hash.
(As discussed in https://github.com/theforeman/foreman/pull/3377#discussion-diff-57880045)
fixes #14415 - check CR availability with Fog, not SETTINGS
Simplifies the code for checking compute resource availability, which
can easily be determined with the registered Fog providers instead of
being stored in SETTINGS in app initialisation. Plugins should always be
loaded, so should not need to override this class method.